The present invention relates to a process for sulfidizing pretreatment of the slag of pyrometallurgical processes, in particular olivine slags and other silicate slags, in order to facilitate the recovery of their valuable metal content during the further treatment stages, especially in froth flotation of slag.
The losses due to the passing of valuable metals, Cu, Ni, Co, Sn, Pb, etc., into slag phases during the smelting of ores and concentrates can be divided into two main categories, i.e. electrochemical and mechanical losses. Electrochemical losses are caused by the physical or chemical solubility, in slag phases, of sulfides and/or oxides which contain valuable metals. Mechanical losses, for their part, are due to the residue of finely-divided metallic and sulfidic drops in the slag phase, resulting either from the slow settling due to high slag viscosity or from adsorption phenomena, or from both of these together. The proportion of the mechanical losses to the total losses is a function of the smelting conditions, the slag phase compositions and the type of the valuable metals. For example, in shaft furnace smelting, percent loss/metal: 60-85/Cu, 97-99/Ni, .about.50/CO, 50-70/Pb. The electrochemical losses can usually be decreased to very low values by regulating the compositions of the slag phases and the oxygen and sulfur potentials of the slags. Decreasing the mechanical losses, on the other hand, is often a very difficult technical problem.
A very great number of different methods have been developed for the recovery of the valuable metals present in smelting plant slags and for decreasing the losses. In this context, primarily those processes linked to a simultaneous reduction and sulfidization of the slag phases are discussed; these processes as such have features in common with the process according to the invention.
The removal of valuable metals from slag phases by means of an addition of sulfide is based on the simultaneous decreasing of both electrochemical and mechanical losses. In this case the objective is that - by means of the addition of sulfide (alone or together with, for example, coke) the oxygen potential of the slag phase is lowered and its sulfur potential is raised, whereby the oxide compounds of the valuable metals are metallized and/or sulfidized. The valuable metals and their compounds dissolve in the excess sulfide phase, which, furthermore, extracts a large quantity of ferric iron in oxide form from the slag melt.
the addition of a common ion to the slag phase lowers its sulfide solubility and, furthermore, the higher solubility of the valuable-metal sulfides in the increased sulfide phase than in the slag phase decreases the losses.
the small metal and matte drops mechanically present in the slag melt combine, forming sufficiently large drops under the effect of the drops of added sulfide, impinging against them.
The sulfide mattes obtained from the sulfide reduction and extraction of slags usually have a low content of valuable metals. Conventionally, sulfide extraction is already linked to the actual smelting. In this case, only low-grade sulfide mattes are produced in the basic smelting apparatus, and the slag melts high in valuable metals, produced during the conversion of these mattes, are returned to the basic smelting unit (e.g. reverberatory furnace: The Physical Chemistry of Copper Smelting, Inst. Min. Met., 1953, London, 11-18, 64-84; Trans. AIME, 1966, 821).
When sulfide mattes high in valuable metals are produced, slag phases with a high oxygen potential and at the same time with a high concentration of oxidic valuable metals are produced. In this case the extraction of the slag by means of a sulfide addition is carried out by discharging, before the extraction, the matte high in valuable metals from the smelting apparatus. In addition to iron sulfide, concentrates low in valuable metals are also used in this case as extracting sulfides (U.S. Pat. No. 2,668,107). By suitable technological means the sulfide extraction can also be arranged to be carried out in connection with the actual production of high-grade matte or metal; however, in this case the slag-extraction part of the process constitutes a zone which produces low-grade matte (U.S. Pat. Nos. 3,460,817, 3,674,463, 3,687,656).
The slag phase obtained in connection with the basic smelting of valuable metals can also be extracted and reduced in a separate converter or electric furnace apparatus. In this case, an effective mixing of the slag melt with the extraction melt, necessary for the process, is essential (U.S. Pat. Nos. 3,506,435, 3,857,700).
The low-grade sulfide mattes produced as products constitute a disadvantage in the processes for extracting molten slag phases. Multi-stage, and therefore expensive, countercurrent extraction apparatus (light-arc furnaces in a series) are necessary for the production of high-grade sulfide mattes. It must be noted that the reduction and sulfidization, as well as the settling of the obtained product sulfide, are time-consuming, and, consequently, high amounts of energy are required for compensating the heat required by the endothermal reactions and the heat losses from the apparatus.
In order to produce the sulfide matte which is a product of extraction carried out at a high temperature, and in order to reach a high equilibrium sulfide solubility of the waste slag, high amounts of sulfide must be used in the extraction processes, which is uneconomical. The slag phases from the production of high-grade sulfide mattes, e.g. 50-60% by weight Cu, still containing a large quantity of iron sulfide, and related converter slags, can be reduced by adding coke, lime and sulfides, to produce, in an electric furnace, a valuable-metal matte of a higher grade than a conventional one. In this case the degree of reduction of the system must already approach the degree of oxidation corresponding to metallic iron (U.N. Seminar on Copper Production, Tashkent, 1970 October; Pyrometallurgical Processes in Nonferrous Metallurgy, Gordon & Breach, 1967, 175).
The various types of froth-flotation processes for slag phases constitute an important group of processes for the recovery of valuable metals from smelting-plant slags. The converter slags which are produced in series conversion of low-grade sulfide mattes obtained from reverberatory smelting and have an advantageous sulfur level are highly suitable for froth flotation, in which case the slag return, which is usually adverse to the basic process, is also eliminated. In addition to the leaching of slag, the flotation methods can also be applied to the slag phases produced in the production of mattes high in valuable metals, provided that the sulfur content in these slags is primarily sufficient (J. Metals, p. 22, 1969, 30; World Mining, 1971, 32-37; Mineral Industries Bull. II, 1968, 1-18). In this case, the slag phase must be cooled slowly in order that its crystallization (and at least partial, diffusive particle growth of the sulfide phase) should take place, thereby improving its grindability. It should be pointed out as one way of applying froth flotation methods that rapidly cooled, coarse-crushed converter slag can be used as grinding pieces in, for example, the milling of copper ores. In this case, when the ore is concentrated by flotation, the valuable-metal contents of converter slags are also recovered, with a high yield (&gt;80%) (Soviet J. Non-Ferrous Metals 7, 1966, 11).
The process according to the present invention primarily comprises a technical-economic improvement relating to the recovery of the valuable metals present in the process slag phases of copper and nickel smelting plants. The process relates to the pyrometallurgical reduction of the slag phase, to the sulfidization of its valuable-metal content, and to the improvement of its grindability when using froth flotation methods.